My own personal favouritites on the bus are:
Cockpit Jenga, similar to one previously mentioned, using the overhead panel, take it in turns switching something off until the other guy bottles it and feels the need to switch it all back on!
Battleships: Really good one for mid pond, using the MCDU and the waypoint function. Pilot 1 enters a waypoint (place bearing dist) from a random wpt apearing on screen. this he calls TARGET. pilot 2 using a different wpt, then has to take a series of shots at the target again using the PBD wpt function. This is usually good for half an hour or until you fill the database up with meaningless wpts.
Also knew a guy who dedicated his life into seeing how quickly he could get the girls to reveal information on how they were waxed (you know the thing R/W strip, hollywood etc) you'd be surprised
